Base station synchronization

ABSTRACT

In a radio network controller (RNC), a covariant matrix and a database are updated. The RNC then detects that a first base station&#39;s time-error-variance exceeds a predetermined threshold. In response, the RNC signals to a user-equipment (UE) to measure a time difference of arrival (TDOA) between signals transmitted from the first base station and a reference base station. The UE measures the requested TDOA between the designated base stations and reports the measurement results back to the RNC. The RNC then compares the measured TDOA with a TDOA value stored in the database. Based on the comparison, the RNC signals to the first base station to adjust its time base.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/075,698,filed on Feb. 14, 2002; now U.S. Pat. No. 6,980,615 which is acontinuation of application Ser. No. 09/826,547, filed on Apr. 5, 2001;now U.S. Pat. No. 6,961,398 which claims priority from ProvisionalApplication Nos. 60/223,405, filed on Aug. 4, 2000 and 60/195,543, filedon Apr. 7, 2000

BACKGROUND

The present invention relates generally to digital communicationsystems. More specifically, the invention relates to a system and methodof synchronizing a plurality of base stations in a cellularcommunication network.

The proposed 3^(rd) generation wireless protocols require an approachthat is based on a simple, but costly procedure of requiring each basestation to be externally synchronized to a highly accurate externalsource.

Techniques which support base station synchronization require that abase station passively listen to synchronization transmissions from itsneighbors, e.g. the synchronization channel (SCH) or the common controlphysical channel (CCPCH), and follow procedures similar to thoseperformed by user equipment (UE) in order to synchronize. Anotherapproach requires each base station to occasionally send a specialsynchronization burst in coordination with one or more of its neighborslistening for the transmission. Yet another approach has UEs measure thetime difference of arrival of transmissions from each of two cells(TDOA). These techniques utilize a precisely accurate source in everybase station. Since each base station has this source, these techniquesare costly and inconvenient.

Therefore, there exists a need for a system and method that allows fast,efficient, and less expensive synchronization between operational basestations without consuming additional physical resources.

SUMMARY

The present invention is a method and apparatus for maintaining basestations synchronized in a wireless communication system. The wirelesscommunication system comprises a radio network controller (RNC), atleast one user-equipment (UE), and a node B, wherein the node Bcomprises a plurality of base stations. In the RNC, a covariant matrixand a database are updated. The RNC then detects that a first basestation's time-error-variance exceeds a predetermined threshold. Inresponse, the RNC signals to a UE to measure a time difference ofarrival (TDOA) between signals transmitted from the first base stationand a reference base station. The UE measures the requested TDOA betweenthe designated base stations and reports the measurement results back tothe RNC. The RNC then compares the measured TDOA with a TDOA valuestored in the database. Based on the comparison, the RNC signals to thefirst base station to adjust its time base.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a communication system.

FIG. 2 is a block diagram of a radio network controller (RNC) made inaccordance with a preferred embodiment of the present invention.

FIG. 3 is a block diagram of a base station and UE made in accordancewith a preferred embodiment of the present invention.

FIG. 4 is an illustration of the hierarchal time quality design made inaccordance with a preferred embodiment of the present invention.

FIGS. 5 a and 5 b is a flow diagram of the system in accordance with apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be describedwith reference to the drawing figures where like numerals represent likeelements throughout.

FIG. 1 illustrates a simplified wireless spread spectrum code divisionmultiple access (CDMA) or time division duplex (TDD) communicationsystem 18. The system 18 comprises a plurality of Node Bs 26, 32, 34, aplurality of RNCs, 36, 38, . . . 40, a plurality of user equipments (UE)20, 22, 24, and a core network 46. A node B 26 within the system 18communicates with associated user equipment 20-24 (UE). The node B 26has a single site controller (SC) 30 associated with either a singlebase station 30, or multiple base stations 30 ₁ . . . 30 _(n). Each basestation has an associated geographic region known as a cell. It shouldbe known that even though base station synchronization is disclosed,cell synchronization may also be accomplished using the presentinvention.

A Group of node Bs 26, 32, 34 is connected to a radio network controller(RNC) 36. The RNCs 36 . . . 40 are also connected to the core network46. For brevity, the following refers to only one node B, but thepresent invention can be readily applied to multiple node Bs.

In accordance with a preferred embodiment, the RNC 36 maintains basestation synchronization within and between the node Bs 26, 32, 34.Referring to FIG. 2, the RNC 36 may request measurements from a basestation 30 ₁ . . . 30 _(n) or UE 20, 22, 24 through its messagegenerator 53; receive measurements through its measure receive device54; optimally update its estimates of states based on these measurementsusing its synchronization controller 55; and manage a set of statesstored in a covariance matrix 57. The stored states are used forsynchronization and represent the time error of each base station 30relative to a reference, the rate of change of each time error, and thetransmission delay between base stations 30.

The RNC 36 also manages a set of measurements stored in a database 59comprising: time of arrival of a measured waveform (i.e. sync burst);time difference of arrival of transmissions from two base stations asmeasured by a UE 20; and estimates of state uncertainties andmeasurement uncertainties. The RNC 36 uses advanced filtering, such asKalman filters, to estimate parameters that define relative clock drift,and to refine parameters such as exact range between one element andanother. The estimated time drift is used to infer the frequencymismatch between the frequency references of the respective basestations and reasonableness checks to ensure that occasional, grosslyinaccurate measurements do not corrupt the process.

The RNC 36 assigns a time quality to each base station 30 ₁ . . . 30_(n). This time quality is measured by the RNC 36 by selecting one basestation as the time base reference for all others. All other basestations are assigned a variable time quality that is updated based onmeasurements and applied corrections. The time quality may be an integer(e.g., 0 to 10). A lower quality value implies a better accuracy.Alternately, the quality may be a continuous (floating point) variable.The reference base station (master base station) is preferably,permanently assigned a quality of 0. All other remaining base stationsare assigned values which vary and are adjusted with respect to thereference base station. To illustrate this time quality hierarchicaldesign, FIG. 4 displays a master base station wherein all base stationsslave 1, slave 2, slave 3, are assigned time quality values which varywith respect to the master base station. In one embodiment the timequality of slave 2 base stations are assigned values which vary withrespect to the slave 1 base stations and slave 3 base stations areassigned values which vary with respect to slave 2 base stations.

The normal mode of operation of the RNC 36 updates a covariance matrix57 for the states stored in the RNC database 59, once per apredetermined time unit (e.g. once per five seconds or a time determinedby an operator). One element of the covariance matrix 57 is theestimated variance of each base station's time error.

When a base station's time error variance exceeds a predeterminedthreshold, the RNC 36 initiates a message to support that base station'stime error update. The update is performed in one of three ways: first,the subject base station is instructed to measure the base station timeof arrival (BSTOA) of a sync burst from a neighboring base station 30 ₁30 ₂ . . . 30 _(n); second, a neighbor base station 30 ₁, 30 ₂ . . . 30_(n) with better quality is instructed to measure BSTOA of the subjectbase station's transmission; or third, a UE 20 measures the BSTOA ofsync bursts of that base stations and a neighboring base station 30 ₁,30 ₂ . . . 30 _(n).

In the first and second approaches using base station to base stationBSTOA, the time of arrival of one base station transmission to anotheris observed. Referring to FIG. 3, a transmitting base station 30 ₁ sendsa known transmission pattern at a predefined time. This transmissionpattern may be a sync burst from the sync burst generator 62 of the basestation 30 ₁, which passes through an isolator 64 prior to beingradiated by an antenna 70. The receiving base station 30 ₁ detects thetransmitted waveform using its measurement device 60 which outputs alarge value when the received signal coincides with the expectedsignature. If the receiver and transmitter were at the same location andhad precisely synchronized clocks, the output of the measurement device60 would occur at the same time as the transmitted waveform. However,clock misalignment and transmission path delay causes a time difference.

Transmission path delay is defined as per Equation 1:R/c+x  Equation 1R/c is the distance, R, between a transmitting unit and receiving unitdivided by the speed of light, c. The term x accounts for equipmentdelays. When base stations are very far apart the quantity, R/ctypically dominates. Radio waves travel at the speed of light,approximately 1 foot per nanosecond, or 3×10⁸ meters per second. Theobjective of base station synchronization is to align the base stationsto within 1-3 microseconds. Therefore, when base stations are separatedby distances on the order of ½ mile (1 km) or more, the distances aresignificant. However, for pico or micro cells, separated by tens ofmeters, the distances are insignificant compared to the measurementaccuracies, x, which dominates.

Based on these considerations, when attempting to synchronize basestations far apart (more than 1 km) the knowledge of the separation isimportant. When attempting to synchronize base stations within 50 metersor so, the exact positions become irrelevant. After the measurement ofBSTOA is performed, the known propagation distance stored in the RNCdatabase 59 is subtracted and the difference is considered themisalignment in time between the base stations.

The third approach measures the relative time difference of arrival(TDOA) between two transmissions sent by two different base stations asobserved by a UE. The UE measures and reports the observed TDOA betweentransmissions from two base stations. The RNC 36 sends a message to theUE 20, 22, 24 to measure the TDOA of two base stations. Upon receipt ofthis message, the UE 20, 22, 24 receives the transmission of the twobase stations, via its antenna 72 and isolator 64, and measures the TDOAusing the UE measure receive device 68 and transmits the measurements toits associated base station.

If the UE position is known (i.e. its range to each of the two basestations r1 and r2 is known) and both base stations timing is correct,the time difference of arrival (TDOA) is defined as per Equation 2.(r1−r2)/c  Equation 2Measured deviations from this value would be an indicator of time basemisalignment. As those skilled in the art know, if the ranges r1 and r2are sufficiently small as would be true for pico-sized cells, it wouldnot be necessary to know their values. Observed time difference ofarrival could be used directly as a measure of time difference oftransmission.

Once an approach is chosen, the appropriate message is transmitted toeither a base station 30 ₁ . . . 30 _(n) or a UE 22, 24, 20. If themessage is sent to a base station 30 ₂, the base station 30 ₂ is toldwhich neighbor to monitor and measure. If the message is to a UE 22, theUE 22 is told which base station to measure in addition to its own basestation.

Referring back to FIG. 2, the RNC 36 has stored the range between eachbase station 30 ₁ . . . 30 _(n) within its database 59. It subsequentlychecks to see if there is a neighbor base station 30 ₁ which has abetter time quality than the base station 30 ₂ to be updated. Once sucha neighbor base station 30 ₁ is found, a message is initiated to theneighboring base station 30 ₁ to take a measurement from the “out ofsync” base station 30 ₂. Alternatively, the RNC 36 is able to send amessage to the “out of sync” base station 30 ₂ and request that it takea measurement of the neighboring base station 30 ₁. The requested basestation, for purposes of this embodiment, the “out of sync” base station30 ₂, then takes the measurement of the “in-sync” base station 30 ₁ andsends the measured value back to the RNC measurement device 54. The RNCmeasurement device 54 forwards the measured value to the synchronizationcontroller 55 which computes the time of transmission of the measurementby subtracting the propagation time R/C.

Once the time of transmission is calculated by the RNC synchronizationcontroller 55, the value is compared to the value stored in the RNCdatabase 59. The RNC synchronization controller 55 then computes Kalmanfilter gains and updates the states in the covariance matrix 57 usingthe difference between the calculated and predetermined time of arrivaland the common gains. If the difference is beyond a certain threshold,the RNC message generator 53 will then send another message to the “outof sync” base station 30 ₂ to adjust its time base or its referencefrequency in order to get “in sync” with the other base station 30 ₃ . .. 30 _(n) under the control of the RNC 36.

The base station 30 ₂ conducts the requested adjustment and reports itback to the RNC measurement device 54. The databases within the RNC 36is updated, including a correction to the subject base station's 30 ₂time reference, its time rate of change, an update of its covariancematrix 57 (including, most significantly, its estimated RMS time errorand drift error), and an update to its time quality. Referring to FIG.4, a base station whose time base is corrected based on a comparison toanother base station, must never be assigned a quality equal to orbetter than that of a base station to which it is a slave to. Thisprocedure guarantees stability. To illustrate, if a slave 2 base stationis to be corrected, the slave 2 base station can only be assigned avalue less than that of time quality of its slave 1 base station. Thisensures that the time quality of a base station will not synchronize toa slave base station of the same level or less which could eventuallylead to a cluster of base stations drifting “out of sync” with themaster base station.

As disclosed earlier, another approach of taking measurements in orderto adjust the “out of sync” base station 30 ₂ uses an UE 20, 22, 24. Ifthis method is chosen by the RNC 36, a message is sent to the UE 22 tomeasure the sync burst of the “out of sync” base station 30 ₂ and the“in sync” base station 30 ₁. Once the measurement is taken by the UE 22,the measurements are sent to the RNC 36 and processed. Similar to themethods described above, the measurements are compared to the knownmeasurements stored in the RNC database 56 and covariance matrix 57 andan adjustment measurement sent to the “out of sync” base station 30 ₂.

The flow diagram of the system in accordance with the preferredembodiment is illustrated in FIG. 5 a and 5 b. The RNC 36 updates thecovariant matrix 57 and database 59 once per unit time (step 501). Whenthe RNC 36 detects that a base station's 30 ₂ . . . 30 _(n) time errorvariance exceeds a predetermined threshold (step 502), the RNC 36decides whether to use a base station to measure BSTOA or a UE tomeasure TDOA in order to update the “out of sync” base station's timeerror variance (step 503). If the RNC 36 decides to measure BSTOA, amessage is sent to a neighboring base station of the “out of sync” basestation to measure the base station time of arrival, or the message issent to the “out of sync” base station to measure the time of arrival ofthe neighboring base station (step 504). The appropriate base stationtakes the necessary measurement (step 505) and transmits the measurementto the RNC 36 (step 506). If the RNC 36 decides to measure TDOA, the RNC36 sends a message to a UE to measure the time difference of arrival oftwo base stations (step 507 a), one being the “out of sync” basestation. The UE measures the TDOA of each base station (step 507 b) andsends the difference of these measurements to the RNC 36 (step 507 c).Upon receipt by the RNC 36 of the appropriate measurements (step 508),the RNC 36 compares the measurement to the value stored in the RNCdatabase 59 (step 509). If the difference is beyond a certain threshold,the RNC 36 sends a message to the “out of sync” base station to adjustits time base or its reference frequency (step 510) in accordance withthis difference. The “out of sync” base station conducts the requestedadjustment (step 511) and reports it back to the RNC 36 (step 512). TheRNC database 59 and covariance matrix 57 are then updated to incorporatethe new values (step 513).

A preferred embodiment is a system and method that resides in each RNC36. In the prior art, a controlling radio network controller (C-RNC)communicates directly with its base stations and a serving radio networkcontroller (S-RNC) communicates directly with its UEs. For cases whereneighboring base stations are under control of different radio networkcontrollers (RNC), there may be a need to add communication between theC-RNCs and S-RNCs that control the neighboring base stations and UEs.

An alternative embodiment requires each pair of base stations that canhear each other to move its own frequency closer to that of the other.The relative amount of adjustment is defined by a set of unique weightswhich are assigned to each base station and stored in the RNC database59. The process of adjusting each of the base stations is the same asdisclosed in the preferred embodiment above except that both the “insync” and “out of sync” base stations are adjusted based on the weightsassigned to the respective base stations. With different weights, onecan achieve varying degrees of centrality, between the fully central tothe fully distributed.

The most preferred embodiment enables an RNC 36 to send time correctionsand/or frequency corrections to a base station 30 ₁ . . . 30 _(n). Themaster base station is responsible to ensure that each of its basestations have a time reference slaved to it, accurate within a specifiedlimit. The RNC 36, in its algorithms and corrections, assumes that thereis negligible error existing between the master base station and itsbase stations and therefore assumes that all base stations have the sametime reference.

As a consequence, the RNC 36 does not attempt to estimate the individualtime errors between the master base station and its base stations andthe master base station must eliminate or compensate for timing errorsbetween the master base station and each of the other base stations,since the associated RNC 36 does not perform a correction. Thisembodiment presents a clean interface between an RNC 36 and a masterbase station. It enables the master base station to apply its ownsolution to slave synchronization which is well suited to pico-cells.

In an alternative embodiment, each base station has an independent timeand frequency reference which enables an RNC 36 to send time correctionsand/or frequency corrections to each base station. The RNC 36, in itsalgorithms and corrections, estimates the states which represent thetime and frequency error of each base station.

As a consequence, the RNC 36 attempts to estimate the individual timeerrors between each base station and the master base station,measurements involving one base station provide no benefit to estimatingthe states of another base station. Therefore, the base stationmanufacturer need only provide loosely bounded errors in the timing andtime drift of the base stations, and every base station must have anacceptable connectivity over the air to another base station (same ordifferent base station).

This alternative embodiment benefits large cellular areas where thedistance between base stations are far. The ability to correct one basestation slaved to the time reference of a master base station throughmeasurements involving another base station slaved to the same masterbase station is limited.

Each base station in this alternative embodiment uses independent timereferences but the master base station provides a frequency reference.An RNC 36 sends time corrections for each base station individuallyand/or a single frequency correction to a master base station. The RNC36 ensures that the clock of each base station is slaved in frequency tothe clock of the master base station. The RNC 36, in its algorithms andcorrections, assumes that there is negligible drift error between themaster base station and its assigned base stations, but estimatesoffsets which are treated as constant.

As a consequence, the RNC 36 estimates the individual time errorsbetween the master base station and its base stations and the commonfrequency drift of the base stations with regard to the master basestation.

This alternative embodiment has features similar to those described inthe previous alternative embodiment where base stations that are farfrom the master base station benefit. This embodiment provides amechanism to remove time mismatches in long distances. Taking advantageof the assumption that these time offsets are stable, this embodimenttakes advantage of a measurement involving any base station slavedfrequency to the clock of the master base station, to update the driftrate for all base stations slaved to the same master base station.

Another alternative embodiment has the RNC 36 providing estimates to themaster base station to support its synchronization of the base stationsslaved to it. An RNC 36 sends time corrections and/or frequencycorrections for each associated base station to its respective masterbase station. The master base station ensures that its associated basestations each have a time reference slaved to itself, accurate within aspecified limit. The master base station may elect to use the basestation-unique estimates to aid in the base station synchronization. TheRNC 36, in its algorithms and corrections, creates a best estimate ofthe time and frequency error between the master base station and itsbase stations. In performing state estimates it weighs the relativeconfidence between the measurements and the base station erroruncertainty.

As a consequence, the RNC 36 attempts to estimate the individual timeerrors between the master base station and its base stations, and themaster base station eliminates and/or compensates for timing errorsbetween the master base station and each base station slaved to its timereference, or requests assistance from the RNC 36.

While the present invention has been described in terms of the preferredembodiments, other variations which are within the scope of theinvention as outlined in the claims below will be apparent to thoseskilled in the art.

1. A method, implemented in a radio network controller (RNC), for maintaining synchronization of base stations, the method comprising: updating a covariant matrix and a database; detecting that a time error variance of a first base station exceeds a predetermined threshold; transmitting a signal to instruct a user equipment (UE) to measure a time difference of arrival (TDOA) between signals transmitted from the first base station and a reference base station; receiving the TDOA measurement; comparing the TDOA measurement with a value stored in the database; and transmitting a message to the first base station to adjust its transmission timing based on said comparison.
 2. The method of claim 1, wherein the covariant matrix maintains a state estimate of each base station associated with the RNC, said state estimates including a time error of each base station relative to a reference, a rate of change of each time error, and a transmission delay between base station pairs.
 3. The method of claim 2, wherein the database maintains previously received TDOA measurement values, and a time reference and time rate of change of each of the plurality of base stations.
 4. The method of claim 3, further comprising determining whether a difference between the measured TDOA value and a TDOA value stored in the database exceeds a predetermined TDOA threshold; and transmitting an adjustment message to the first base station if said difference exceeds said predetermined TDOA threshold.
 5. The method of claim 4, further comprising: receiving an adjustment report, wherein the adjustment report includes that a transmission timing of the first base station has been adjusted according to the transmitted adjustment message; and updating the database and covariant matrix according to said adjustment.
 6. The method of claim 5, wherein the reference base station is a master base station in a node B, and wherein the first base station is slaved to said master base station.
 7. An RNC configured to maintain base stations synchronized, wherein said base stations are within a node B, the RNC comprising: a covariant matrix configured to maintain a state estimate of each of a plurality of base stations, and wherein said state estimates include a time error of each base station relative to a reference, a rate of change of each time error, and a transmission delay between base station pairs; a database configured to maintain received TDOA measurement values between signals transmitted from a first base station and a reference base station; a message generator configured to generate and transmit measurement-request messages; a measurement receive device configured to receive and process received TDOA measurement values; and a synchronization controller configured to compute a time of transmission of each of the base stations and filter gains based on received TDOA measurement values.
 8. The RNC of claim 7, wherein the synchronization controller is further configured to determine whether a difference between the received TDOA value and a TDOA value stored in the database exceeds a predetermined threshold.
 9. The RNC of claim 8, wherein the message generator is further configured to generate and transmit adjustment messages according to a determined difference between the received TDOA value and the stored TDOA value.
 10. The RNC of claim 9, wherein the measurement receive device is further configured to receive and process reporting messages responsive to transmitted adjustment messages.
 11. The RNC of claim 10, wherein the synchronization controller is further configured to update the state estimates in the covariance matrix and update the database with received TDOA values. 